The present invention relates to a semiconductor laser device capable of emitting a plurality of laser beams of different wavelengths, and a method for fabricating the same.
In recent years, there is an increasing demand for semiconductor laser devices in many industrial fields, and active researches and development have been performed primarily for semiconductor laser devices including a group III-V compound semiconductor layer, particularly a compound semiconductor layer containing GaAs or InP.
In the field of optical information processing, systems have been realized in which data is written or read with a semiconductor laser device which includes an AlGaAs layer and oscillates an infrared laser beam having a wavelength in a 780 nm band. Such semiconductor laser devices have already been widely used in the field of compact disks (CDs), etc.
A recording apparatus for use with a medium such as an magneto-optical disk having a greater capacity than that of a CD employs a semiconductor laser device which includes an AlGaInP layer and oscillates a laser beam in a 680 nm band shorter than a 780 nm band.
Recently, a digital video disk (DVD) system capable of long-time playback of high-definition images requires a semiconductor laser device which emits a red laser beam having a wavelength in a 650 nm band. Thus, it has been attempted to improve the recording density of an optical disk through the reduction in oscillation wavelength.
Some DVD apparatuses for reading DVD data have compatibility with CDs which allows for reading data of both DVDs and CDs so that one can utilize conventional CD data as well as DVD data. Therefore, the light source of the pickup head device of such a DVD apparatus needs to be provided with two semiconductor laser devices, including a first semiconductor laser device which includes an AlGaAs layer and emits an infrared laser beam in a 780 nm band, and a second semiconductor laser device which includes an AlGaInP layer and emits a red laser beam in a 650 nm band.
In such a case, if an optical processing system is provided for each of the semiconductor laser devices, it is necessary to provide an optical system for combining the laser beam in a 780 nm band with the laser beam in a 650 nm band, thereby complicating the structure of the pickup head device and imposing a limit on the downsizing of the pickup head device.
In view of this, a hybrid type semiconductor laser device in which two semiconductor laser devices are arranged adjacent to each other, or a monolithic type semiconductor laser device in which two semiconductor laminated structures are provided in parallel to each other on a single substrate has been proposed in the art (see Japanese Laid-Open Patent Publication No. 11-186651 and Proc. of the 60th Fall Meeting of JSAP, 3a-ZC-10).
FIG. 21 illustrates an example of a conventional monolithic type semiconductor laser device. The semiconductor laser device includes a first semiconductor laminated structure 2 including an AlGaInP layer and a second semiconductor laminated structure 3 including an AlGaAs layer which are provided on a single semiconductor substrate 1, emitting a laser beam in a 650 nm band from a light-emitting spot 4 of the first semiconductor laminated structure 2 and emitting a laser beam in a 780 nm band from a light-emitting spot 5 of the second semiconductor laminated structure 3.
With hybrid type or monolithic type semiconductor laser device as described above, it is no longer necessary to provide an optical system for combining two laser beams of different wavelengths, thereby allowing for simplification and downsizing of the pickup head device.
However, in the hybrid type semiconductor laser device, the pitch of the two semiconductor laser devices is influenced by the width dimension of each semiconductor laser device. As a result, the pitch of the light-emitting spots is on the order of 100 xcexcm or more.
In the monolithic type semiconductor laser device, it is necessary to provide two semiconductor laminated structures on a single semiconductor substrate. As a result, the pitch of the light-emitting spots is on the order of 10 nm or more due to the limit of the process for separating the two semiconductor laminated structures from each other.
An optical pickup head device needs to have a half mirror for directing an emitted laser beam toward the optical disk, an object lens for focusing the laser beam having passed through the half mirror into a spot on the optical disk, a photodetector for detecting the laser beam reflected from the optical disk, etc.
However, since the object lens has been downsized along with the downsizing of the optical pickup head device, the focusing characteristic of the object lens varies due to the variation in the laser beam incident point on the object lens (the position on the object lens where the laser beam is incident varies because there are two different light-emitting spots). As a result, it is difficult to focus the laser beam having passed through the object lens into a tiny spot on the optical disk.
Moreover, when the angle at which the laser beam having passed through the object lens is incident upon the optical disk varies, the direction in which the laser beam is reflected from the optical disk also varies, thereby making it necessary to provide two photodetectors.
In view of the above-mentioned conventional problems, the present invention has been devised for the purpose of emitting a plurality of laser beams of different wavelengths from a single light-emitting spot or two immediately adjacent light-emitting spots, thereby realizing reliable focusing of the plurality of laser beams of different wavelengths into a tiny spot on the optical disk and detection of the plurality of laser beams of different wavelengths with a single photodetector.
In order to achieve the above-described object, a semiconductor laser device according to the present invention includes: a first semiconductor laminated structure which is provided on a front-side region of a substrate and includes a first active layer for oscillating a first laser beam having a first wavelength band; and a second semiconductor laminated structure which is provided on a rear-side region of the substrate and includes a second active layer for oscillating a second laser beam having a second wavelength band, wherein an emission direction of the first laser beam and an emission direction of the second laser beam are same.
In the semiconductor laser device according to the present invention, the first semiconductor laminated structure for oscillating the first laser beam is provided on the front-side region of the substrate, and the second semiconductor laminated structure for oscillating the second laser beam is provided on the rear-side region of the substrate, wherein the emission direction of the first laser beam and the emission direction of the second laser beam are same. Therefore, the first and second laser beams having different wavelengths can be emitted from a single light-emitting spot or two immediately adjacent light-emitting spots in the front surface of the first semiconductor laminated structure which is provided on the front-side region. Thus, it is possible to realize reliable focusing of a plurality of laser beams of different wavelengths into a tiny spot on an optical disk and detection of the plurality of laser beams of different wavelengths with a single photodetector.
In the semiconductor laser device according to the present invention, it is preferred that the emission direction of the first laser beam and the emission direction of the second laser beam are collinear with each other.
In this way, the first and second laser beams having different wavelengths can be emitted from a single light-emitting spot in the front surface of the first semiconductor laminated structure.
In the semiconductor laser device according to the present invention, it is preferred that a light-emitting spot of the second laser beam is above or below a light-emitting spot of the first laser beam.
In this way, the first and second laser beams having different wavelengths can be emitted from two immediately adjacent light-emitting spots in the front surface of the first semiconductor laminated structure. Since the pitch between the first light-emitting spot and the second light-emitting spot is not influenced by the width dimension of the semiconductor integrated structure, it is possible to reduce the pitch between the light-emitting spots to be 1 xcexcm or less.
Moreover, the rear surface of the first active layer can be either a transmissive surface or an absorptive surface, thereby increasing the freedom in selecting the optical design conditions. When the energy gap of the first active layer is greater than the energy gap of the second active layer, and the optical axis of the first laser beam and the optical axis of the second laser beam are aligned with each other, the front surface of the second active layer, i.e., the rear surface of the first active layer, becomes an absorptive surface for the first laser beam. Normally, the energy gap of a semiconductor layer above or below the second active layer is greater than the energy gap of the second active layer. Therefore, if the optical axis of the second laser beam is above or below the optical axis of the first laser beam, the front surface of the semiconductor layer above or below the second active layer, i.e., the rear surface of the first active layer, can be either a transmissive surface or an absorptive surface for the first laser beam.
When the emission direction of the second laser beam is above or below the emission direction of the first laser beam, it is preferred that an energy gap of a semiconductor layer in the second semiconductor laminated structure which opposes a rear surface of the first active layer is greater than an energy gap of the first active layer.
In this way, the semiconductor layer in the second semiconductor laminated structure which opposes the rear surface of the first active layer transmits the first laser beam therethrough, thereby reducing the loss of the first laser beam.
When the emission direction of the second laser beam is above or below the emission direction of the first laser beam, it is preferred that the first semiconductor laminated structure includes a first cladding layer located between the substrate and the first active layer and a second cladding layer located above the first active layer; the second semiconductor laminated structure includes a third cladding layer located between the substrate and the second active layer and a fourth cladding layer located above the second active layer; and a composition of the first cladding layer and a composition of the third cladding layer are same, or a composition of the second cladding layer and a composition of the fourth cladding layer are same.
In the semiconductor laser device according to the present invention, it is preferred that an energy gap of the first active layer is greater than an energy gap of the second active layer.
In this way, the second laser beam is not absorbed while it propagates through the first semiconductor laminated structure, and thus is reliably emitted from the front surface of the first semiconductor laminated structure.
Especially when the optical axis of the first laser beam and the optical axis of the second laser beam are aligned with each other, since the second active layer has a large absorption coefficient for the first laser beam oscillated in the first active layer, the first laser beam is oscillated with the front surface of the first semiconductor laminated structure and the front surface of the second semiconductor laminated structure serving as resonator surfaces. Moreover, since the first active layer is transparent to the second laser beam oscillated in the second active layer, the second laser beam is oscillated with the front surface of the first semiconductor laminated structure and the rear surface of the second semiconductor laminated structure serving as resonator surfaces. Therefore, two laser beams of different wavelength bands can be reliably emitted from a single light-emitting spot.
In the semiconductor laser device according to the present invention, it is preferred that the first active layer contains indium and phosphorus; and the second active layer contains gallium and arsenic.
In this way, the first laser beam has an oscillation wavelength of about 650 nm and is a red laser beam, and the second laser beam has an oscillation wavelength of about 780 nm and is an infrared laser beam. Thus, it is possible to obtain a semiconductor laser device which is optimal for use in a pickup head device of a DVD apparatus.
In the semiconductor laser device according to the present invention, it is preferred that a front surface of the first semiconductor laminated structure is coated with a non-reflection coating layer; and a rear surface of the second semiconductor laminated structure is coated with a high-reflection coating layer.
In this way, two laser beams of different wavelength bands can be reliably emitted from the front surface of the first semiconductor laminated structure.
In the semiconductor laser device according to the present invention, it is preferred that the semiconductor laser device further includes a dielectric member between a rear surface of the first semiconductor laminated structure and a front surface of the second semiconductor laminated structure; and the dielectric member has a refractive index which is between an effective refractive index of a stripe region of the first active layer and an effective refractive index of a stripe region of the second active layer.
In this way, the insulation between the first semiconductor laminated structure and the second semiconductor laminated structure can be ensured by the dielectric member. Moreover, the optical coupling efficiency between the first laser beam and the second semiconductor laminated structure is improved, and the optical coupling efficiency between the second laser beam and the first semiconductor laminated structure is also improved, thereby improving the optical characteristics of the semiconductor laser device.
A first method for fabricating a semiconductor laser device according to the present invention is a method for fabricating a semiconductor laser device including: a first semiconductor laminated structure which is provided on a front-side region of a substrate and includes a first active layer for oscillating a first laser beam having a first wavelength band; and a second semiconductor laminated structure which is provided on a rear-side region of the substrate and includes a second active layer for oscillating a second laser beam having a second wavelength band, the method including the steps of: growing a first tentative semiconductor laminated structure having the same laminated structure as the second semiconductor laminated structure on the substrate; removing a front-side portion of the first tentative semiconductor laminated structure, thereby producing the second semiconductor laminated structure on the rear-side region of the substrate; growing a second tentative semiconductor laminated structure having the same laminated structure as the first semiconductor laminated structure on the front-side region of the substrate and on the second semiconductor laminated structure; and removing a portion of the second tentative semiconductor laminated structure above the second semiconductor laminated structure, thereby producing the first semiconductor laminated structure on the front-side region of the substrate.
With the first method for fabricating a semiconductor laser device according to the present invention, it is possible to reliably fabricate a monolithic type semiconductor laser device, wherein the first semiconductor laminated structure for oscillating the first laser beam is provided on the front-side region of the substrate, and the second semiconductor laminated structure for oscillating the second laser beam is provided on the rear-side region of the substrate, and wherein the emission direction of the first laser beam and the emission direction of the second laser beam are same.
A second method for fabricating a semiconductor laser device according to the present invention is a method for fabricating a semiconductor laser device including: a first semiconductor laminated structure which is provided on a front-side region of a substrate and includes a first active layer for oscillating a first laser beam having a first wavelength band; and a second semiconductor laminated structure which is provided on a rear-side region of the substrate and includes a second active layer for oscillating a second laser beam having a second wavelength band, the method including the steps of: growing a first tentative semiconductor laminated structure having the same laminated structure as the first semiconductor laminated structure on the substrates removing a rear-side portion of the first tentative semiconductor laminated structure, thereby producing the first semiconductor laminated structure on the front-side region of the substrate; growing a second tentative semiconductor laminated structure having the same laminated structure as the second semiconductor laminated structure on the rear-side region of the substrate and on the first semiconductor laminated structure; and removing a portion of the second tentative semiconductor laminated structure above the first semiconductor laminated structure, thereby producing the second semiconductor laminated structure on the rear-side region of the substrate.
With the second method for fabricating a semiconductor laser device according to the present invention, it is possible to reliably fabricate a monolithic type semiconductor laser device, wherein the first semiconductor laminated structure for oscillating the first laser beam is provided on the front-side region of the substrate, and the second semiconductor laminated structure for oscillating the second laser beam is provided on the rear-side region of the substrate, and wherein the emission direction of the first laser beam and the emission direction of the second laser beam are same.
A third method for fabricating a semiconductor laser device according to the present invention includes: a first step of providing a first laser chip including a first active layer for oscillating a first laser beam having a first wavelength band and a second laser chip including a second active layer for oscillating a second laser beam having a second wavelength band; and a second step of fixing the first laser chip to a front-side region of a substrate and fixing the second laser chip to a rear-side region of the substrate, wherein the second step includes the step of fixing the first laser chip and the second laser chip so that an emission direction of the first laser beam and an emission direction of the second laser beam are same.
With the third method for fabricating a semiconductor laser device according to the present invention, it is possible to reliably fabricate a hybrid type semiconductor laser device, wherein the first laser chip for oscillating the first laser beam is provided on the front-side region of the substrate, and the second laser chip for oscillating the second laser beam is provided on the rear-side region of the substrate, and wherein the emission direction of the first laser beam and the emission direction of the second laser beam are same.
In the first to third methods for fabricating a semiconductor laser device, it is preferred that the emission direction of the first laser beam and the emission direction of the second laser beam are collinear with each other.
In this way, the first and second laser beams of different wavelengths can be emitted from a single light-emitting spot in the front surface of the first semiconductor laminated structure or the first laser chip.
In the first to third methods for fabricating a semiconductor laser device, it is preferred that the emission direction of the second laser beam is above or below the emission direction of the first laser beam.
In this way, the first and second laser beams having different wavelengths can be emitted from two immediately adjacent light-emitting spots in the front surface of the first semiconductor laminated structure. Since the pitch between the first light-emitting spot and the second light-emitting spot is not influenced by the width dimension of the semiconductor integrated structure, it is possible to reduce the pitch between the light-emitting spots to be 1 xcexcm or less.
In the first to third methods for fabricating a semiconductor laser device, it is preferred that an energy gap of the first active layer is greater than an energy gap of the second active layer.
In this way, the second laser beam is not absorbed while it propagates through the first laser chip, and thus is reliably emitted from the front surface of the first semiconductor laminated structure or the first laser chip.
Especially when the optical axis of the first laser beam and the optical axis of the second laser beam are aligned with each other, since the second active layer has a large absorption coefficient for the first laser beam oscillated in the first active layer, the first laser beam is oscillated with the front surface of the first semiconductor laminated structure or the first laser chip and the front surface of the second semiconductor laminated structure or the second laser chip serving as resonator surfaces. Moreover, since the first active layer is transparent to the second laser beam oscillated in the second active layer, the second laser beam is oscillated with the front surface of the first semiconductor laminated structure or the first laser chip and the rear surface of the second semiconductor laminated structure or the second laser chip serving as resonator surfaces. Therefore, two laser beams of different wavelength bands can be reliably emitted from a single light-emitting spot.
In the first to third methods for fabricating a semiconductor laser device, it is preferred that the first active layer contains indium and phosphorus; and the second active layer contains gallium and arsenic.
In this way, the first laser beam has an oscillation wavelength of about 650 nm and is a red laser beam, and the second laser beam has an oscillation wavelength of about 780 nm and is an infrared laser beam. Thus, it is possible to obtain a semiconductor laser device which is optimal for use in a pickup head device of a DVD apparatus.
The first to third methods for fabricating a semiconductor laser device preferably further include the steps of: coating a front surface of the first semiconductor laminated structure with a non-reflection coating layer; and coating a rear surface of the second semiconductor laminated structure with a high-reflection coating layer.
In this way, two laser beams of different wavelength bands can be reliably emitted from the front surface of the first semiconductor laminated structure or the first laser chip.
The third method for fabricating a semiconductor laser device preferably further includes, after the second step, the step of filling a gap between a rear surface of the first laser chip and a front surface of the second laser chip with a dielectric member having a refractive index which is between an effective refractive index of a stripe region of the first active layer and an effective refractive index of a stripe region of the second active layer.
In this way, the insulation between the first laser chip and the second laser chip can be ensured by the dielectric member. Moreover, the optical coupling efficiency between the first laser beam and the second laser chip is improved, and the optical coupling efficiency between the second laser beam and the first laser chip is also improved, thereby improving the optical characteristics of the semiconductor laser device.
A fourth method for fabricating a semiconductor laser device according to the present invention includes: a first step of providing a first laser chip including a first active layer for oscillating a first laser beam having a first wavelength band, a second laser chip including a second active layer for oscillating a second laser beam having a second wavelength band, and a third laser chip including a third active layer for oscillating a third laser beam having a third wavelength band; and a second step of fixing the first laser chip to a front-side region of a substrate, fixing the second laser chip to a central region of the substrate, and fixing the third laser chip to a rear-side region of the substrate, wherein the second step includes the step of fixing the first laser chip, the second laser chip and the third laser chip so that an emission direction of the first laser beam, an emission direction of the second laser beam, and an emission direction of the third laser beam are same.
With the fourth method for fabricating a semiconductor laser device according to the present invention, it is possible to reliably fabricate a hybrid type semiconductor laser device, wherein the first laser chip for oscillating the first laser beam is provided on the front-side region of the substrate, the second laser chip for oscillating the second laser beam is provided in the central region of the substrate, and the third laser chip for oscillating the third laser beam is provided on the rear-side region of the substrate, and wherein the emission direction of the first laser beam, the emission direction of the second laser beam and the emission direction of the third laser beam are same.
In the fourth method for fabricating a semiconductor laser device, it is preferred that the emission direction of the third laser beam is collinear with the emission direction of the first laser beam or the emission direction of the second laser beam.
In this way, the third laser beam and the first or second laser beam having different wavelengths can be reliably emitted from a single light-emitting spot in the front surface of the first laser chip.
In the fourth method for fabricating a semiconductor laser device, it is preferred that an energy gap of the first active layer is greater than an energy gap of the second active layer; and the energy gap of the second active layer is greater than an energy gap of the third active layer.
In this way, the second laser beam is not absorbed while it propagates through the first laser chip, and the third laser beam is not absorbed while it propagates through the first and second laser chips. Thus, the second and third laser beams are reliably emitted from the front surface of the first laser chip.
In the fourth method for fabricating a semiconductor laser device, it is preferred that the first active layer contains gallium and nitrogen; the second active layer contains indium and phosphorus; and the third active layer contains gallium and arsenic.
In this way, the first laser beam is a blue laser beam, the second laser beam is a red laser beam, and the third laser beam is an infrared laser beam. Thus, three laser beams of different oscillation wavelengths can be emitted from a single light-emitting spot or two immediately adjacent light-emitting spots. As a result, it is possible to realize a three-wavelength semiconductor laser device capable of accommodating three types of optical disks of different standards.
The fourth method for fabricating a semiconductor laser device preferably further includes, after the second step, the steps of: filling a gap between a rear surface of the first laser chip and a front surface of the second laser chip with a first dielectric member having a refractive index which is between an effective refractive index of a stripe region of the first active layer and an effective refractive index of a stripe region of the second active layer; and filling a gap between a rear surface of the second laser chip and a front surface of the third laser chip with a second dielectric member having a refractive index which is between the effective refractive index of the stripe region of the second active layer and an effective refractive index of a stripe region of the third active layer.
In this way, the insulation between the first laser chip and the second laser chip and the insulation between the second laser chip and the third laser chip can be ensured. Moreover, the optical coupling efficiency between each laser beam and each laser chip is improved, thereby improving the optical characteristics of the semiconductor laser device.